A compressed air control apparatus is known from German Patent No. DE 44 21 575 C2 corresponding to European Patent No. EP 0 689 117 B1. A pressure controller, an air dryer and a multi circuit protection valve are arranged in a common housing of the known compressed air control apparatus. The pressure controller includes an outlet valve. The pressure controller including the outlet valve controls the load phase and the idle phase of the apparatus. The air dryer serves to remove humidity from the compressed air being delivered by a compressor. The known multi circuit protection valve includes a plurality of overflow valves having limited backflow and a plurality of solenoid valves. Usually, each one of the overflow valves is associated with one of the circuits of the multi circuit protection valve. Each one of the solenoid valves is associated with each one of the overflow valves. The solenoid valves are designed and arranged to arbitrarily operate the overflow valves. Each solenoid valve is supplied with compressed air by a central aerating system, and it includes its own deaerating connection. In the currentless condition of the solenoid valve, the deaerating connection is closed. A separate regeneration circuit serves for regeneration of the air dryer. Another solenoid valve is arranged in the regeneration circuit. A regeneration conduit coming from the solenoid valve is arranged to bypass a check valve leading to the central aerating system. Consequently, the regeneration circuit leads in a backward direction through the air dryer to reach the outlet of the outlet valve. The solenoid valve of the regeneration circuit is designed to be closed in its currentless condition such that the regeneration conduit is connected to the atmosphere by its own deaerating connection. The check valve being located in the regeneration conduit serves to prevent continuous loss of compressed air. It is also possible to associate one common solenoid valve with the two brake circuits. The control conduit of the common solenoid valve leads to the two overflow valves of the two brake circuits. The known compressed air control apparatus requires the use of a comparatively great number of solenoid valves.
Another compressed air control apparatus for compressed air systems of motor vehicles is known from German Patent No. DE 195 44 621 C1 corresponding to European Patent No. EP 0 776 807 B1. The known compressed air control apparatus includes a pressure controller, an air dryer and a multi circuit protection valve together forming a common structural unit. Each circuit of the multi circuit protection valve is associated with an overflow valve having limited backflow. Each overflow valve includes a valve body or a so called “movable wall” separating a stream chamber from a rear chamber including a spring. The stream chamber (or the flow chamber) is connected with the rear chamber by a bypass conduit in which an electrically controllable solenoid valve having its own deaerating connection is located. The solenoid valve is designed to be closed in its currentless condition. This means that it locks the connection in the bypass conduit. On the other hand, when the solenoid valve is excited, it realizes the connection, and it locks the deaerating connection. The dryer may be regenerated in a regeneration phase. A separate solenoid valve is located in a regeneration conduit.